a) Field of the invention
The present invention relates to an image transmission optical system that may be used in optical instruments, such as flexible endoscopes and non-flexible endoscopes equipped with distal ends, which are adapted so as to insert cylindrical members into locations permitting no direct observation and allow observation of images of objects from positions apart from the objects by transmitting the images through the cylindrical members.
b) Description of the prior art
There is known the optical system disclosed by Japanese Patent Kokoku Publication No. Sho 49-5993 shown in FIG. 1 as an image transmission optical system to be used in non-flexible endoscopes and so on. This image transmission optical system uses bar-shaped lens components 1 and 1' which are arranged symmetrically with regard to a plane P as a lens unit for relaying an image. This lens unit functions to focus an image I of an object again as another image I'. This image transmission optical system focuses the image of object I into the image I' by using the lens unit as an image relaying unit and relays the image of the object from the left side to the right side in FIG. 1 by repeating the relaying operation described above. An image transmission optical system capable of relaying an image for a desired distance can be composed by arranging the lens unit consisting of the bar-shaped lens components 1 and 1' used as the relaying unit in a required number. Further, in the image transmission optical system, surfaces 1a and 1a' having positive refractive powers serve as the visual field lenses for sequentially transmitting a pupil even when the unit lens system for image relaying (the above-mentioned lens unit) is used in a large number.
The above-described image transmission optical system has a simple composition, but is insufficient in the aberration correcting capability thereof, whereby said optical system allows curvature of field and astigmatism to be produced remarkably and constitutes a cause for degradation of image, especially when said optical system is used in a long non-flexible endoscope which requires a large number of image relaying operations.
In order to correct this defect, there is known another image transmission optical system consisting of a lens unit having the composition illustrated in FIG. 2. In this image transmission optical system, a pair of meniscus lens components 2 and 2', which have concave surfaces opposite to each other, are arranged between the bar-shaped lens components 1 and 1' shown in FIG. 1, and the lens unit is arranged symmetrically with regard to the plane P. This optical system is adapted in such a manner that negative curvature of field and negative astigmatism are corrected by the meniscus lens components which are opposite to each other. However, this optical system comprises a large number of surfaces kept in contact with air, i.e., eight air-contact surfaces per lens unit. Accordingly, reflections on these air-contact surfaces pose a problem. Speaking concretely, loss of light due to reflections results in reduction of amount of light when an image is relayed in a large number of times and an image is colored due to variation of spectral reflectance of the respective reflecting surfaces. Further, the image transmission optical system comprises a large number of lens components, and requires tedious procedures and a long time for assembly. In addition, since imaging performance of the optical system is dependent more largely on eccentricity of the meniscus lens components than eccentricity of the other lens components, even a slight manufacturing or assembling error constitutes a cause for undesirable local blurring, eclipse of pupil, etc. There are known image transmission optical systems which use aspherical surfaces for solving the problem described above as exemplified by the image transmission optical system disclosed by Japanese Patent Kokai Publication No. Sho 57-207215 shown in FIG. 3. This image transmission optical system comprises lens units, each consisting of two bar-shaped cemented lens components each of which is composed of a bar-shaped biconvex lens element having a relatively high refractive index for favorably correcting Petzval's sum, and two negative lens elements made of materials having relatively low refractive indices respectively and cemented to both the ends of said lens element. In this image transmission optical system, the spherical aberration produced by the cemented surfaces (spherical surfaces) is corrected by the aspherical surfaces formed as the air-contact surfaces of the lens elements having the negative refractive power.
In this image relaying optical system, however, aberrations are produced by the lens surfaces and imaging performance is degraded when radii of curvature are enlarged on the convex surfaces of the bar-shaped biconvex lens element for obtaining positive refractive power of each lens component. It is therefore necessary to obtain positive refractive power of each lens component without enlarging so much the radius of curvature on the bar-shaped lens element, and refractive power of the bar-shaped lens element must be strengthened very much. Further, this conventional image relaying optical system has a composition which hardly permits correcting astigmatism and spherical aberration at the same time. FIG. 4 shows curves illustrating aberration characteristics calculated on the basis of the numerical data listed in Table 2 of Japanese Patent Kokai Publication No. Sho 57-207215. As is understood from these curves, astigmatism is corrected sufficiently but correction of spherical aberration is not sufficient in the image transmission optical system disclosed by the above-mentioned publication. Since aberrations produced by each relay lens unit are added as an image is sequentially transmitted by relay lens units in an image transmission optical system, it is undesirable to use the above-mentioned conventional image optical system in non-flexible endoscopes.